The present invention relates to a method of removing a surface coating, in particular to a method of removing an indium tin oxide coating from a substrate for an electroluminescent display.
Electroluminescence is the emission of light by a material when subjected to an electric field.
A typical thick film phosphor electroluminescent device comprises a layer of electroluminescent material in a dielectric matrix, sandwiched between two planar conducting electrodes. The electroluminescent material comprises phosphor particles, typically a zinc sulphide (ZnS) powder doped with manganese (Mn), microencapsulated in a dielectric material. Typically, silver- or graphite-loaded screen-printable inks, and indium tin oxide (ITO), a transparent conductive material, respectively are used to form the electrodes on a substrate such as a polyester film. When an AC voltage is applied between the electrodes, the electroluminescent material emits light.
The inventors have recently developed thick film electroluminescent displays in which a plurality of shaped independent electrodes are provided on at least one side of a layer of electroluminescent material. A voltage may be applied selectively to each of these independent electrodes to illuminate a respective region of the display. A thick film electroluminescent display is created by selecting the configuration of the independent electrodes to represent information, for example in the form of a seven-segment display or the like.
A problem associated with the manufacture of thick film electroluminescent displays is that the independent electrodes must be connected electrically to a voltage source for the display. In a convenient manufacturing technique, electrical connections are applied as conductive tracks on the rear surface of the device, for example by screen printing conductive ink. However, when a voltage is applied to the conductive tracks, they can act as electrodes causing the electroluminescent material to illuminate. Such illumination is undesirable as it detracts from the clarity of the display.
One way to prevent unwanted illumination due to the conductive tracks is to remove the ITO from the substrate in the regions opposite the conductive tracks. The ITO should be removed completely and in register with the other layers of the display.
Commercially available materials suitable for use as substrates in electroluminescent displays can have very resilient coatings of ITO. The manufacturers of such materials recommend the use of an acid for etching away the unwanted areas of the ITO coating. According to the recommended process, the coated substrate is initially provided with an etching mask, which is typically alkali soluble, and the substrate is immersed in acid, for up to 20 minutes, depending on the grade of ITO. The substrate is then rinsed and the mask is removed with an alkali solution. The process is messy, and difficult to control.
An alternative method of removal is laser ablation of the ITO layer. However, this method is slow because the laser is scanned across the surface of the substrate, and the process can take a very long time where ITO is to be removed from a relatively large area.